1. Field of the Invention
This invention relates to a manufacturing unit for semiconductor devices, and in more particular to a manufacturing unit for semiconductor devices in which a wafer is secured using vacuum adsorption and an etching solution is dropped or sprayed on the wafer while rotating or spinning it to thereby perform etching of a thin metallic film (for example, a thin aluminum film)(From here on the unit will be called a spin etcher).
2. Description of the Prior Art
To describe prior spin etchers, an example of a spin etcher for a thin aluminum film will be used and described in reference to the drawing. FIG. 1 is a drawing showing the construction of a prior aluminum spin etcher. A thin aluminum film is formed on a wafer 3 and will be used as a mask when forming a wiring and implanting impurities into the wafer 3, and on top of this thin aluminum film a photoresist masking pattern is formed in order to obtain the prescribed aluminum pattern using etching. The wafer 3 is carried by a wafer chuck 11 and secured using vacuum adsorption. The aluminum etching process of the wafer 3 is performed by using a spin motor 10 to rotate the wafer chuck 11 and rotate the wafer 3 while at the same time spraying an aluminum etching solution on the wafer 3 using an etching solution spray nozzle 4. The end of the aluminum etching process is judged by monitoring the status of the aluminum etching using an end-point detection sensor. When etching aluminum, a photosensor which uses infrared rays with a wavelength of approximately 950 nm is used as the end-point detection sensor.
Next, the end-point detection mechanism will be described. First, the infrared rays generated by a light emitting diode inside the photoelectric element 5 of the end-point detection sensor travel along an optical fiber cable 8 and are projected from the light projector 7 of the end-point detection sensor onto the rear surface of the wafer. Aluminum has the characteristic of absorbing infrared rays and so the amount of infrared rays that penetrates the wafer changes depending on the status of the aluminum etching. The infrared rays that penetrate the wafer 3 pass along the optical fiber cable 8 from the light receiver 6 of the end-point detection sensor and are converted into photoelectric current by a phototransistor inside the photoelectric element 5 of the end-point detection sensor, and then sent as an electric signal to the etching-end-point judgement circuit 9. In the etching-end-point judgement circuit 9, a drop in the voltage is monitored by using the photoelectric current, and the end point of the etching is judged by the voltage change for a unit time. When the end point of the etching has been judged, the rotation of the spin motor 10 stops and the spray of the etching solution from the etching solution spray nozzle 4 stops, and the etching process ends.
In the prior spin etcher described above, while the wafer is rotating during the etching process, the light which penetrates the wafer is detected by the end-point detection sensor which is fixed in an arbitrary location, therefore the location of the etching pattern on the wafer where the status of the etching is monitored constantly changes with the rotation of the wafer, and the waveform of the end-point detection signal obtained from the photosensor is very unstable and so there is a problem in that it is very easy for the etching-end-point judgement circuit to misjudge the end point of the etching.
When judging the end point of the etching for aluminum etching, the photosensor monitors weak voltage changes which are 1% or less of the initial voltage and it requires an end-point detection signal waveform that has become stable. Aluminum etching normally advances at a rate of 80 angstroms per second and so if the end point of the etching is misjudged, the dimensions of the aluminum pattern are far off the designed dimensions and this causes various characteristic errors in semiconductor chips and causes a reduction in reliability.